A more robust system, requiring 120 or more satellites, could cost as much as $500 billion or several trillion dollars. Furthermore, space systems -- such as the Space Shuttle and International Space Station -- have often grossly overrun their budgets, due to underestimation during the budgeting process and unforeseen technical hurdles encountered during development and construction. For the sake of argument, $1 trillion for a significant space weapons capability is a reasonable estimate. All of this presages adverse impacts on other national security programs. As an accountant for any national treasury might put it, the books simply do not balance. The United States far out-spends any other state in the area of national security, and its annual defense budget of $350 billion is mostly committed upon arrival. Given salaries, pensions, benefits, facilities, operations and maintenance, only $125 billion is left for procurement and R&D. Within that, major acquisitions of the F-22, joint strike fighter, bombers, carrier battle groups, UAVs, etc., all vie for this limited funding. Every other national defense establishment operates under similar or more severe funding constraints. In the context of military spending alone, space-based weapons are simply unaffordable. Beyond this, one country's pursuit of space weapons could well catalyze the next major arms race.

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Deblois, Bruce M. "The Advent of Space Weapons." Astropolitics. Vol. 1, No. 1 (Summer 2003). [ 15 quotes ]